1. Field of the Invention
This invention relates to industrial platforms and, more particularly, to plastic pallets having intumescent properties that provide the pallets with fire resistance characteristics.
2. Description of the Prior Art
Wooden stringer pallets are the preferred materials of pallet construction within the North American distribution system. Four hundred (400) million new or refurbished wooden pallets are introduced into a distribution system comprising 1.9 billion pallets each year, according to the US Forest Service.
Plastic pallets have been used to replace wood pallets with some degree of success over the past several years. Plastic pallets have a low market share however because they suffer from one significant disadvantage in that they are considerably more expensive than a comparable wooden pallet. Thermoplastic materials constitute a significant proportion of the total cost of a plastic pallet, and a given amount of relatively expensive plastic material is required to produce a pallet with a measure of load-bearing strength that is comparable to wooden pallets. Therefore, the plastics industry is attempting to overcome the initial price difference that exists between wooden and plastic pallets, so that the plastics industries can gain more market share.
Approximately 4 to 6 percent of the annual North American production of pallets are in the form of plastic pallets. Increasing the strength while utilizing less material is an important object of the plastics industry. The plastic industry however has reached a plateau. Only marginal, rather than significant break through in increased strength to weight ratios have been anticipated using conventional methods of the plastics industry.
The twin sheet thermoforming sector of the plastics industry has captured a share of the plastic pallet market disproportionate to its share of the overall plastics industry. Accordingly, it may be suggested that the art of thermoforming is a competitively and comparatively advantageous starting point for the development of new break through plastic pallet methodologies.
The “standard” 48×40-inch wooden stringer pallet has a dynamic load bearing performance specification of 2,800 pounds. This load bearing specification is the benchmark against which plastic pallets are compared. In order to meet this specification in thermoformed plastic, a combination of two (×2) twin sheet pallet members have been proposed. Two twin sheet members are combined to provide what in known in the material handling industry as a rackable plastic pallet.
Conventional rackable twin sheet pallet designs comprise a load supporting platform and a load-distributing base. Three common techniques are used by thermoforming practitioners to join the load supporting platform and the load distributing base in a fixed spaced apart relationship for the introduction of fork lift tines and the like for movement and storage of the plastic pallet within the distribution system.
A first method characterized in U.S. Pat. No. 5,413,052 to Breezer et al., utilizes a plurality of separately molded blocks to maintain the twin sheet members forming the deck and the base of the pallet a fixed distance apart. A second method characterized in U.S. Pat. No. 5,117,762 to Shuert suggests a load supporting platform with a plurality of depending legs to maintain the twin sheet pallet members a fixed distance apart. In yet another method, two pallet members are fused together where corresponding mirror image projecting elements upon each member come together, as in U.S. Pat. No. 5,401,347 to Shuert.
Each method characterized presents problems. In the first methodology, an undesirable plurality of mechanical fasteners and molded elements are required. In the second method, the load-bearing surface of the platform has pockets forming the leg projections, which reduces the surface area available for supporting a load. In the third method, where the two members are fused together, the arrangement is disadvantageously permanent. These approaches are not satisfactory. A low cost means of coupling and de-coupling the members of a racking style pallet is needed.
In order to meet the 2,800-pound load bearing benchmark it has also been necessary to encapsulate metal frame structures between the twin sheets comprising the thermoformed pallet members. U.S. Pat. No. 5,404,829 to Shuert illustrates in FIG. 7 how the top sheet of thermoplastic forming the load support deck includes elements that depend downward from the surface to capture reinforcing beams.
In the U.S. Pat. No. 5,413,052 execution of a reinforced pallet no depending elements on the load-bearing surface are suggested. A substantially uninterrupted surface is preferred over a relatively stronger developed surface having several pockets or depressions. The deck member of '052 would however be unsatisfactory for supporting loads without the reinforcing cross members because this structure would be considerably weaker than a deck with a developed surface structure. Accordingly, a mold combination that can produce either a strong non-reinforced or an exceptionally strong reinforced pallet without interruptions on the load-supporting surface would be advantageous and is therefor needed.
Plastic pallets must also provide a level of fire resistance that is at least equal to or better than wooden pallets should a fire occur within the warehouse setting. Plastic pallets will not substitute wooden pallets on a large scale if plastic pallets create hazards that prevent a fire from being extinguished. A plastic pallet that creates more fire hazards than a wooden pallet will necessitate fire protection upgrades, including increased sprinkler systems and insurance premiums that could become very costly to the plastic pallet user. According to this problem, one pallet known as the GE Extreme™ Pallet has been offered.
The GE Extreme™ Pallet is UL classified and Factory Mutual approved to meet the National Fire Code (NFPA 13) for commodity and idle storage of pallets. Although this particular plastic pallet has been used to some advantage, it is nonetheless heavy weight (approx. 57.5 pounds) and is constructed of plastic materials made from expensive General Electric Company Noryl® and Xenoy® resins. The problem is that these resins are considerably more expensive than the commodity resins of the olefin group such as polyethylene and polypropylene, which are the preferred materials for constructing low cost plastic pallets.
A number of methodologies have been used in the past to provide fire retardant polyolefin compositions, as for example in electrical wiring. These prior art methods may be known by referring to U.S. Pat. No. 3,810,862 to Mathis et al, U.S. Pat. No. 5,356,983 to Vijayendran et al. and U.S. Pat. No. 5,946,878 to Grund et al. A first problem with these methods is that the materials are relatively expensive as they are used throughout the article's resinous composition. A second problem is the resultant loss of the physical properties and general processability of the carrier resin forming the article.
Coatings have also been proposed to provide protective fire retardant properties to plastic structural articles, and may be understood by referring to U.S. Pat. No. 5,924,589 to Gordon and U.S. Pat. No. 6,110,559 to De Keyser. An intumescent coating system comprising a first layer providing a breakthrough barrier and a second layer providing thermal insulation has also been proposed, as in U.S. Pat. No. 5,989,706 to McGinniss et al. Problems with coating systems are that they require secondary manufacturing operations and materials which can be expensive to acquire and apply and they would be subject to damage/removal in a rough pallet handling environment.
It is known that thermoformable resins can be co-extruded to yield an engineered sheet construction with enhanced characteristics. For example, U.S. Pat. No. 5,143,778 to Shuert proposes a co-ex sheet construction to provide a more rigid pallet structure. The co-ex principle has been suggested by Gordon in U.S. Pat. No. 5,984,126 to provide an industrial container formed from a structural sheet that has an outer layer of fire resistant intumescent material to prevent the breaching and subsequent spilling of flammable lading. Although the Gordon approach may be useful in some applications, it would be difficult to implement the approach in a twin sheet pallet that would typically be under load.
Polyolefins have a notoriously low heat deflection temperature and a co-ex intumescent twin sheet pallet construction would surely collapse when softened by the heat of a fire. It is also not known what intumescent admixture Gordon proposes. Another problem being that an intumescent system must be processable by the practitioner of thermoforming methods. According to these problems, there is need for a new and useful approach to provide a fire resistant pallet that will also maintain it load bearing strength in high temperature environments.
It may also be appreciated that conventional wooden pallets are low-tech. Plastic pallets are becoming increasingly sophisticated. A hollow pallet having an internal wireless communications device that triggers a 911 emergency data signal in response to a fire or the heat of a combustion flame to a remote “emergency” monitor would be beneficial.
It is also understood that plastic pallets have been used to replace wooden pallets with some success because wooden pallets deteriorate through normal wear and tear. Examples of wooden pallet deterioration include, but are not limited to, splintered wooden boards and stringers and projecting nails. In addition to causing damage to packaging materials and automated pallet handling equipment, these examples of deterioration also cause workforce injuries as a result of manual wooden pallet handling. While plastic pallets eliminate these problems to a large extent and have been used to some advantage because they do not deteriorate in the same fashion, it may be argued that plastic pallets remain nonetheless difficult to manually handle by warehouse workers because of their heavyweight construction. Pallets in the prior art have not been developed with ergonomic principles in mind. Ergonomic pallets are needed.
It is also known that plastic pallets, which are used to support loads that may be suspended upon racks adjacent the work area of a warehouse worker, are often times constructed of plastic materials that exhibit low coefficients of friction. Two such materials with relatively low coefficients of friction include polyethylene and polypropylene. According to this potential safety problem it has been advantageous to offer such pallet materials with skid resistant properties or treatment.
U.S. Pat. No. 4,428,306 discloses a non-skid surface applied to the polyethylene sheet prior to forming the pallet structure. Alternatively, in U.S. Pat. No. 5,648,031, it has been suggested anti-slip droplets may be sprayed upon the surface of the material forming the plastic pallet to provide a skid-resistant treatment. Although these and other approaches provide some skid resistant protection they are disadvantageous in that they required additional material and or processing expense in their original manufacture and eventual recycling. Pallets with a high coefficient of friction surface on the top and the bottom are needed to prevent slippage of the load carried by the pallet, and slippage of the pallet on the support surface.
It is also known that plastic pallets must interface within distribution networks where it is common to unitize a pallet load with shrink-wrap and other banding materials. Plastic pallets have not been adequately developed to interface with these and other packaging methods. In U.S. Pat. No. 5,676,064 to Shuert, a downward extending peripheral lip and indents in the outer leg structures are suggested to accommodate packaging materials.
U.S. Pat. No. 5,408,937 to Knight et al. discloses indented surfaces upon the legs are suggested to receive wrapping materials. Although these arrangements are helpful, they do not allow the warehouse worker to manually and ergonomically initiate the starting stretch and cling of widely used packaging films around the pallet for final unitization. A pallet amenable to unitization is needed.
As above discussed, it is well known in the art that plastic pallets and particularly thermoformed plastic pallets have many advantages over wooden pallets. The disadvantage of initial price, however, is increasingly a more complex justification for selecting wooden pallets when compared to plastic pallets. Although twin sheet plastic pallets have been employed successfully to replace wood, breakthroughs in the cost equation and the value-added execution of thermoformed plastic pallets are finally needed to justify a wholesale conversion from wooden pallets to plastic pallets.